Go for a drive on the highway, get up into your top gear and reach an RPM of about 2000 or more. Note the RPM and Speed. The speed needs to be the actual speed according to a GPS as your speedo may not be correct unless you have stock everything.

Then use this equation where Tire diameter is (inches), RPM is as displayed on your tach, and speed is in the actual mph according to a GPS.

Go for a drive on the highway, get up into your top gear and reach an RPM of about 2000 or more. Note the RPM and Speed. The speed needs to be the actual speed according to a GPS as your speedo may not be correct unless you have stock everything.

Then use this equation where Tire diameter is (inches), RPM is as displayed on your tach, and speed is in the actual mph according to a GPS.

Gear Ratio = [(RPM x 3.14 x Tire Diameter x 60) / (12*5280)] / Speed

Only true when the transmission is not in Overdrive. I.e. in 4th gear if it's a 5-speed, 5th gear if it's a 6-speed, or after turning the Overdrive off if it's the 42RLE 4-speed automatic.

Go for a drive on the highway, get up into your top gear and reach an RPM of about 2000 or more. Note the RPM and Speed. The speed needs to be the actual speed according to a GPS as your speedo may not be correct unless you have stock everything.

Then use this equation where Tire diameter is (inches), RPM is as displayed on your tach, and speed is in the actual mph according to a GPS.

Gear Ratio = [(RPM x 3.14 x Tire Diameter x 60) / (12*5280)] / Speed

That method is only accurate when the transmission is not in Overdrive. I.e. in 4th gear if it's a 5-speed, 5th gear if it's a 6-speed, or after turning the Overdrive off if it's the 42RLE 4-speed automatic.

That method is only accurate when the transmission is not in Overdrive. I.e. in 4th gear if it's a 5-speed, 5th gear if it's a 6-speed, or after turning the Overdrive off if it's the 42RLE 4-speed automatic.

My jeep isnt fancy enough to have overdrive, i was only blessed with three gears

Hey i was wondering why i was getting such bad mpgs? Im getting around 10 mpg, i have a 02 with 4.0 and auto transmission. I have a 3.25 suspension lift with 33" tires, could it be that i only have 3.07 gears? I am getting close to 120,000 miles and the spark plugs i dont think have ever been changed. Im using 10w30 oil every 3000 miles and the air filter is clean. Any help to help me get better mpgs would be apreciated, and if your rig's set up is somewhat identical to mine tell me what your getting.

You have gotten some great info so far. Have you changed your fluids? I have been going through my new to me 02 4.0/auto and doing maintenance on everything. Recently I installed the deeper transmission pan on the 32RH. The new fluid made a difference at the pump. I have also started using an ethanol stabilizer in hopes of neutralizing the effects of that crap, it really does kill fuel mileage.

I had one code, evedently, my engines suckin in air somewhere its not supposed to, must be a hole somewhere in my vent, going to try to locate it tommarrow. Also going to get the price of plugs and will talk to my mechanic about regearing the next time i see him

Hey i was wondering why i was getting such bad mpgs? Im getting around 10 mpg, i have a 02 with 4.0 and auto transmission. I have a 3.25 suspension lift with 33" tires, could it be that i only have 3.07 gears? I am getting close to 120,000 miles and the spark plugs i dont think have ever been changed. Im using 10w30 oil every 3000 miles and the air filter is clean. Any help to help me get better mpgs would be apreciated, and if your rig's set up is somewhat identical to mine tell me what your getting.

Wow so many different options on what to check. Its simple first change the spark plugs nothing too fancy just some NGK or champion plugs make sure to gap them to .035. replace the wires and if your jeep has a cap and rotor setup than replace them too. this will be the cheapest option see if it improves the economy and power you get.

Depends on the state of the fuel system. You might want to pull the fuel rail and injectors and clean them out. It couldn't hurt. I've been reading about injector upgrades for a few months and most who post about changing them out talk about gunk build up in the rail. Maybe a restriction, poor atomization of fuel, etc... Maybe have a caliper dragging or engine control issues from bad sensors, or restricted cats. Have you had any luck with a diagnosis or fix? Post it up and let us know before the corn cobs start flyin

I had one code, evedently, my engines suckin in air somewhere its not supposed to, must be a hole somewhere in my vent, going to try to locate it tommarrow. Also going to get the price of plugs and will talk to my mechanic about regearing the next time i see him

Everyone knows Jeeps do not get good MPG compared to regular SUV or cars. They need to be kept in top tune to get the best mileage they can possibly get.

I just went down this path. Used to get 15 mpg and it dropped to 11 recently. It has been hotter this year then ever and I have had the A/C on all the time so that does not help. But I suspected something else was also wrong? I replaced everything, full tune-up w/Mopar parts, Mopar 02 sensor (had Bosch), new distributor (bad bushing), cam position sensor, checked cat converter, replaced muffler. The old muffler was smaller and had smaller diameter pipes going in. Maybe it was a little plugged or something? Surprisingly I got +2 mpg with a new muffler. With the muffler I got up to 13 mpg, still not right?

I also checked obvious locations for vacuum leaks and found nothing.

Yesterday I replaced all of my old injectors that had 160K on them with Bosch 4-port injectors. One was leaking.

I also replaced my valve cover gasket. This is where I found something weird. All of the studs that hold down the valve cover were hand loose! The factory uses RTV only to seal no gasket. That works but over time the RTV must shrinks away. The thickness of the RTV bead shrinks so the valve cover is no longer tight. This might be a possible source of a vacuum leak?

Depends on the state of the fuel system. You might want to pull the fuel rail and injectors and clean them out. It couldn't hurt. I've been reading about injector upgrades for a few months and most who post about changing them out talk about gunk build up in the rail. Maybe a restriction, poor atomization of fuel, etc... Maybe have a caliper dragging or engine control issues from bad sensors, or restricted cats. Have you had any luck with a diagnosis or fix? Post it up and let us know before the corn cobs start flyin

Another very good place to check. A bad caliper that is sticking or dragging can kill mileage.